There has been considerable effort devoted in recent years to solving various ecological and environmental problems such as air pollution, acid rain, etc. Combustion effluents and waste products from various sources are a major source of air pollution when discharged into the atmosphere. Unless the waste products are treated to remove deleterious components, the degradation of the environment will continue. Acid rain, forest and vegetation decline, changes in the ozone layer, harmful and irritating smog, etc., are examples of the results of the pollution of the atmosphere.
The common sources of pollution include internal combustion engines, industrial plants, utility boilers, gas turbines, and commercial establishments such as service stations, dry cleaners, etc. It has been estimated that power plants are responsible for about 1/3 of the annual NO.sub.x emissions while mobile sources such as automobiles and trucks are responsible for about 40% to about 50%. The types of air pollutants generated by such facilities include particulate emissions such as coal ash, sulphur compounds such as SO.sub.2 and SO.sub.3, carbon monoxide, ozone, and nitrogen oxides, commonly referred to collectively as "NO.sub.x ".
One of the common components found in polluted air is nitrogen dioxide (NO.sub.2) which is known to be an extremely poisonous material. Nitrogen dioxide is introduced into the atmosphere from the various sources such as commercial plants producing nitric acid, but a major source of nitrogen dioxide is from nitric oxide (NO) formed by combustion processes of the types described above. The nitrogen oxide is formed during such combustion processes by (1) the reaction of nitrogen with atmospheric oxygen in the high temperature portion of the flame ("thermal fixation); and (2) the oxidation of organic nitrogen compounds in the fuel on burning. The nitric oxide formed on combustion is converted to nitrogen dioxide on contact with air in the atmosphere.
Various procedures have been suggested to remove the oxides of nitrogen from waste gases so that the gases may be discharged into the atmosphere without harm to the environment. Nitrogen oxide(s) emissions from boilers, gas turbines and internal combustion engines have been reduced by modifying the design of the engine or boiler to be more efficient or to operate at a lower temperature. Other proposals for reducing nitrogen oxide emissions involve use of various chemicals to reduce the nitrogen oxide content of effluent gases by converting the nitrogen oxides to innocuous gases. Such chemical processes, however, generally require extremely high temperatures such as in the range of about 1600.degree. to about 2000.degree. F. and higher. The temperatures of some of these chemical reactions for reducing nitrogen oxide content have been reduced by utilizing catalysts which are effective in promoting the reduction of nitrogen oxide. Processes using a catalyst to promote the reduction of nitrogen oxide are generally referred to as selective catalytic reduction ("SCR") processes. Using a catalyst in the effluent gas streams has certain disadvantages such as the expense of the catalyst, the life of the catalyst, the expense and difficulty of contacting the combustion effluents with the catalyst, etc. Accordingly, there has been continued emphasis on procedures for reducing nitrogen oxide emissions which do not involve the direct use of catalysts. Processes for reducing nitrogen oxide content of effluent gases which do not use a catalyst in the effluent gas stream are referred to in the art as selective non-catalytic reduction ("SNR") processes.
Various techniques for reducing NO.sub.x emissions from various combustion processes are described in the article entitled "Reducing NO.sub.x Emissions,"Power Sep. 1988, pp S-1 to S-13. Among the chemicals which have been suggested as being useful in reducing the nitrogen oxide content of combustion effluents are nitrogen-containing compounds such as ammonia, urea, cyanuric acid, etc. For example, U.S. Pat. Nos. 3,900,554; 4,335,084; 4,743,436; 4,849,192; and 4,851,201 describe processes utilizing ammonia to reduce nitrogen oxide emissions.
The use of urea is described in U.S. Pat. Nos. 4,208,386; 4,325,924; 4,719,092; and 4,851,201. The use of cyanuric acid, and more specifically, the decomposition product of cyanuric acid, isocyanic acid, for reducing the nitrogen oxide content of combustion effluents is described in U.S. Pat. Nos. 4,731,231; 4,800,068; 4,861,567; and 4,908,193; and by R. A. Perry and D. L. Siebers, Nature Vol. 324, 18/25, pp 657-658, Dec. 18, 1986. Perry proposes that isocyanic acid (HNCO) is formed from the decomposition of cyanuric acid when cyanuric acid is heated above about 330.degree. C. When the isocyanic acid is mixed with the exhaust gas stream at temperatures 400.degree. C. or higher, a series of reactions is proposed to occur that results in the loss of HCNO and NO. U.S. Pat. No. 4,908,193 contains one claim (claim 21) to a method of reducing the NO content of a gas stream by contacting the gas stream with NCO radicals provided the NCO radicals have not been generated by addition directly to said NO-containing gas stream per se of solid cyanuric acid particles of a diameter of 0.1 to 10 mm.
U.S. Pat. Nos. 4,743,436 and 4,849,192 describe the process for treating effluent gases containing nitrogen oxides, sulfur trioxide, etc., wherein the effluent gas is first treated with ammonia to reduce the nitrogen oxide content and thereafter with methanol to reduce the sulfur trioxide content of the combustion effluent to SO.sub.2 thereby minimizing the formation of ammonium bisulfate and sulfuric acid.
Japanese Patent 54-28771 discloses the addition of particles with cyanuric acid within the range of 0.1 to 10 mm. in diameter to exhaust gases to remove NO.sub.x. Temperatures of from 600.degree. to 1500.degree. C. are disclosed with the preference expressed for temperatures of 1200.degree.-1300.degree. C. A surface reaction is postulated.
The decomposition products obtained from the photodissociation of HNCO in the vacuum ultraviolet is the subject of an article by H. Okabe in the I. Chem. Phys., Vol. 53, No. 9, pp. 3507-15, (Nov. 1, 1970). Previous authors had suggested that the product of the photodissociation of HNCO would include compounds such as NH, NCO, CO, NH.sub.2, N.sub.2 and H.sub.2. The HNCO utilized in Okabe's studies was prepared from cyanuric acid powder. Okabe concluded that the product of the photodissociation of HNCO in the vacuum ultraviolet was NCO and NH. The role of NCO radicals as intermediates in fuel bond nitrogen conversion was discussed by R. A. Perry in I. Chem. Phys., 82, 5485-88 (Jun. 15, 1985). Perry summarized the work of previous investigators relating to the production of NCO radicals and subsequent reactions of NCO in flames, and Perry noted that Cookson et al, Ber. Bunsenges, Phys. Chem., 89, 335 (1985) had reported measuring the room temperature rate for the reaction of NCO with NO. Perry suggested three possible reaction schemes for the reaction of NCO with NO as follows:
(A) NCO + NO = N.sub.2 O + CO; PA0 (B) NCO + NO = N.sub.2 + O + CO; and PA0 (C) NCO + NO = N.sub.2 + CO.sub.2. PA0 (A) generating NCO free radicals, and thereafter PA0 (B) adding the free radicals to the gas whereby the NCO free radicals react with the nitrogen oxide in the gas to form nitrogen and carbon dioxide. The NCO free radicals may be obtained from a variety of sources such as by the catalytic decomposition of cyanuric acid; the reaction of formaldehyde with nitrogen or nitric oxide; the reaction of carbon monoxide with nitrogen oxide or nitrogen or mixtures thereof; the reaction of methanol with nitrogen in the presence of a catalyst, etc. The NCO free radicals thus generated are effective for reducing the nitrogen oxide content of post combustion gases in accordance with the process of the present invention at temperatures from ambient temperature up to about 2000.degree. F.
Perry concluded from his study of the temperature rate constant of the reaction that the reaction of NCO with NO proceeds in accordance with reaction (A). The value obtained by Perry was reported to be in agreement with the value obtained by Cookson et al for the same reaction. Perry further postulated that reaction (C) was unlikely. Perry also concluded that the reaction of NCO with NO may be an important source of nitrous oxide formation in those flames where NCO and NO are present in sufficient concentration. The Perry article does not relate to any process for treating exhaust gases, and does not disclose generating NCO free radicals and thereafter adding the free radicals to an exhaust to react with the NO.sub.x in the post-combustion gases in the exhaust.